1. Field of the Invention
The present invention relates to the field of photolithography, and more particularly to a method for the controlled aging of a photoresist such as a mid-ultraviolet (mid-UV) or deep-ultraviolet (DUV) photoresist so as to obtain an aged photoresist that has faster photospeeds. Faster photospeeds are important in the field of the photolithography since they require lower radiation doses to pattern the photoresist. Lower doses, in turn, equate to decreased processing cost.
2. Background of the Invention
Photoresists such as mid-UV and DUV are typically employed in the semiconductor industry to provide a desired pattern to a surface of a substrate needing patterning. The patterning of the substrate is achieved by applying a photoresist to a surface of a substrate, patterning the photoresist by exposing the same to a defined dosage of radiation, developing the patterned photoresist and transferring the pattern from the photoresist to the substrate via an etching process such as reactive-ion etching. As known to those skilled in the art, the development step includes the use of an appropriate developer solution that is used to selectively remove the photoresist in either the exposed regions (positive-tone photoresists) or, in the unexposed regions (negative-tone photoresists). The steps of applying a photoresist, patterning the photoresist and developing the pattern are referred to in the art as photolithography or simply as lithography.
A common problem with prior art photoresists especially mid-UV and DUV photoresists is that prior art photoresists suffer from variation in photospeeds. That is, a first photoresist may have a different photospeed compared to a second photoresist, despite the two photoresists having the same composition and being made from the same manufacturer. This variation in photospeeds is problematic to a photolithographer since it requires adjustment in the radiation dose used during the photolithographic process and requires rework of the entire photo-tooling process.
The adjusted radiation dose and rework of the photo-tooling process needed to overcome the above-mentioned problem with prior art photoresists adds additional time and cost to the overall photolithography process. This additional cost, in turn, is passed along to the consumer when purchasing a device made from prior art photolithography processing.
In view of the state of prior art photoresists, there is a continued need for developing a new and improved method wherein the variation in photospeeds between a given lot of photoresists is substantially eliminated. Moreover, a method is needed in which a given lot of photoresists have been properly aged to achieve a targeted photospeed that does not vary from run to run.
The present invention provides a method for the controlled aging of a photoresist such as a mid-UV and DUV photoresist so as to provide an aged photoresist that has a desired targeted photospeed that does not vary from run to run. Specifically, the inventive method comprises the step of:
aging a solution containing at least a photoresist resin composition at a temperature below the thermal decomposition of said photoresist resin composition, but not below 20xc2x0 C., for a time period that is effective in achieving a targeted photospeed that is above the photospeed of an unaged photoresist.
In accordance with the present invention, the aging step may be conducted in air, in an inert gas ambient such as N2, He, or Ar, or under a vacuum. Moreover, the photoresist resin composition employed in the present invention preferably includes any mid-UV or deep UV, positive-tone or negative-tone photoresist. The term xe2x80x9cmid-UVxe2x80x9d as used herein denotes a photoresist that may be activated by irradiating the same with UV radiation having a wavelength of about 365 nm, whereas the term xe2x80x9cDUVxe2x80x9d denotes a photoresist that may be activated by irradiating the same with UV radiation having a wavelength of about 248 nm. Although mid-UV and DUV photoresists are preferred in the present invention, the inventive aging method works with all other photoresists such as, for example, 157 nm photoresists, 193 nm photoresists, e-beam photoresists and G-line photoresists. The term xe2x80x9cpositive-tonexe2x80x9d photoresist denotes a photoresist whose exposed regions are removed, i.e., developed, using an appropriate developer solution, whereas the term xe2x80x9cnegative-tonexe2x80x9d photoresist denotes a photoresist whose unexposed regions are removed, i.e., developed using an appropriate developer solution.
The inventive method provides an aged photoresist having a faster photospeed than heretofore possible with unaged photoresist. Moreover, the entire lot of aged photoresist would have substantially the same photospeed thereby eliminating the variation problem that is typically observed with prior art photoresists.